The invention pertains to the field of forming concrete structures, such as walls, by slip forming. When forming elongated structures of concrete it has long been recognized that a traveling mold or form can be employed to economically shape such concrete structures. Concrete pipe has been formed by slip molding processes for many years, as have street curbs and the like. Concrete curbs and gutters may also be formed by slip forms and walls may likewise be so constructed as long as the wall is of a relatively low height, and able to retain its form once released from the mold, and prior to the concrete hardening. Typical concrete slip form devices for conduits are shown in U.S. Pat. Nos. 3,106,760; 3,551,537 and 3,562,056. Apparatus for slip forming curbs and gutters are shown in U.S. Pat. Nos. 2,818,790; 3,161,117; 3,175,478 and 3,261,272.
In the formation of concrete structures of considerable height, such as a wall, by using slip form techniques, the form or screed, as it is often designated, consists of a pair of parallel sidewalls and is open at the front and rear ends. The form is mounted upon a suitable wheeled support for continuous movement during formation of the wall, and a hopper is associated with the front end of the screed into which concrete is supplied. The concrete is normally subjected to virbration in the hopper and as it passes through the form in order that the concrete will completely fill the form cross section, and as the form shapes the wall, the wall will maintain the configuration determined by the form sidewalls and harden. Of course, when slip forming a wall of a height of two or three feet, for instance, the concrete introduced into the form is very viscous and "dry" in order that its "flow" characteristics are minimized and its ability to be free standing is high.
The lower edges of the slip form normally either engage the ground, or the foundation, upon which the concrete structure is to be supported, and must be close enough to the ground or foundation to prevent the concrete from flowing under the sidewalls.
Of course, the configuration of the sidewalls of the screed may be as desired in order to form a wall having a given cross-sectional configuration. In effect, a concrete structure formed by slip forming is "extruded" from the slip form, and the cross-sectional configuration thereof is determined by the configuration of the screed adjacent its open rear end.
In the construction of highway expressways and freeways the median often includes a barrier wall dividing the highway lanes bearing traffice flowing in opposite directions. In particular, such barrier walls are employed in urban areas wherein the median is of minimum width and a barrier is required to separate adjacent lanes of traffic traveling in opposite directions. Such barrier walls, in the past, normally consisted of steel or wooden posts inserted into the ground upon which guard rails are mounted. Concrete barrier walls are now being increasingly used by highway departments, and such walls are often formed by concrete slip forming apparatus.
The conventional cross-sectional configuration of concrete highway barrier walls includes a wide base which usually is defined by vertical sides, the configuration then tapers upwardly and inwardly, and at the third stage tapers upwardly at a decreased rate to the upper wall edge. This particular type of configuration has been designed to provide sufficient strength to resist impact forces and has also been designed to provide optimum "rebound" characteristics, as well as minimize the concrete requirements and produce an aesthetically pleasing and mechanically efficient structure. Concrete barrier walls, when of a longitudinally linear configuration, are usually symmetric with respect to a central vertical plane, assuming the grade and highway level on each side of the wall to be the same. However, in instances wherein the grade or highway line on opposite sides of the wall is different, the configuration of the wall sides must vary to accommodate the difference in grade lines. Thus, it is not uncommon for a highway concrete barrier wall to be asymmetrical in cross section with respect to the wall central vertical plane. Such asymmetrical barrier wall configurations can be produced by shaping the sidewalls of the form or screed as desired. However, as the configuration of the screed sidewalls are fixed, it is not possible to vary the wall configuration during wall formation.
Previously, it has not been possible to use concrete highway barrier walls economically in those instances where the highway defines a curve, or otherwise departs from a linear or straight length. As a highway defines a curve the highway or road surface is "banked" to compensate for centrifugal forces imposed upon the vehicle as it travels through the curve and a barrier wall constituting the median of a divided highway on a curve will separate different grade or road lines due to the banked configuration of the highway. As one side of the barrier wall is adjacent the outer edge of the inside highway, and as the other side of the barrier wall defines the inner edge of the outer highway, a difference of 12 inches or more in the grade line on opposite sides of the barrier wall often exists. As it is important that at all locations on the curve that the configuration of the lower regions of the barrier wall be constant with respect to the adjacent road surface, it is necessary to change the barrier wall cross-sectional configuration in proportion to the radius of the curve being defined. Thus, it will be appreciated that as most highway curves constitute a plurality of radii, it is not possible to form a highway barrier wall of varying transverse cross-sectional asymmetrical configuration by conventional slip form apparatus. Thus, on curves, or where the transverse cross-sectional configuration of the barrier wall must be continually varied, concrete barrier wall has not been formed heretofore by slip molds or screeds, or if concrete barrier wall is used on curves it is formed by conventional stationary forms by laborious hand methods, resulting in very expensive installations.